Put reflective strips (you know, like you find on some of these newfangled shoes, and on firemen's coats) on the corners of carts. That way, they stand out even in the darkest of parking lots, provided, of course, you have yer headlights on.

Retroreflectivity is different than just reflectivity. While most objects are reflective to some extent or other, they tend to scatter the light in all directions. Mirrors reflect the light to the...uh...you know how mirrors reflect light. Retroreflective materials reflect light *only* in the direction of the source (plus a few degrees around the axis). Even mirrors don't do that.

I love this idea. Nearby customers of the market I frequent are permitted to remove shopping carts from the premises and return / reuse them on their next spree. For some this requires crossing several lanes of traffic in the dark. (+)